The invention relates to a crank-connecting rod mechanism for an internal combustion engine or the like.
The present crankshaft as used in all standard piston engines is already several decades old and its principle has remained unchanged. Although constant refinements have resulted in a highly reliable and trouble-free crank mechanism, the mechanism clearly has its limitations.
The object of the present invention is to provide a crank-connecting rod mechanism which offers surprising new possibilities.
In order to accomplish that objective, the present invention provides a crank-connecting rod mechanism for an internal combustion engine or the like, which comprises a shaft rotating about its axis, a first crank fixed to said shaft, a crank pin fixed to said first crank, which extends parallel to but at a first radial distance from said shaft, a second crank rotatably mounted on the first crank pin, which is fixed to a second crank pin having an axis at a second radial distance from the axis of the first crank pin, a connecting rod which is rotatably mounted on the second crank pin, as well as rotary means to force the second crank to rotate relative to the first crank upon rotation of the shaft.
In certain uses, significant benefits can be obtained from the addition of an additional crank comprising rotary means which determine the rotation of the second crank relative to the first crank.
In the case of two-stroke engines, for example, it is advantageous if the rotary means are so arranged that the second crank rotates at the same speed, while in an opposite direction, as the first crank, in which case the first radial distance is preferably the same as the second radial distance.
The use of such a crank-connecting rod mechanism makes it possible to have the connecting rod move straight up and down, in that the two crank pins rotating in opposite directions neutralize each other""s movements in a direction perpendicularly to the connecting rod whilst enhancing said movements in the direction of the connecting rod. The connecting rod thus has linear bearings. This offers several advantages. In the first place, hardly any transverse forces will occur between the piston and the cylinder wall (the so-called guideway forces), due to the fact that the connecting rod moves straight up and down. The risk of seizing pistons and the friction between piston wall and cylinder wall are thus reduced. Further, a close fit rather than the use of a piston spring could provide a seal between the piston and the cylinder, which reduces the need for lubrication. The mechanism can furthermore be completely balanced for each individual cylinder, so that a low-vibration operation is possible with a one-cylinder engine as well. The construction makes it possible to use a simple suction membrane, wherein the so-called xe2x80x9cfalse volumexe2x80x9d (between membrane and the underside of the piston) can be minimized, as a result of which suction and filling are enhanced. Further, a connecting rod which does not move outwards in a lateral direction enables a longer stroke of the piston, wherein the port design can be optimized. The suction volume can be influenced by means of a suitable selection of the diameter of the connecting rod, by which the compression/expansion ratio can be determined. Finally, the construction is very advantageous for an opposed cylinder arrangement, wherein a piston can be mounted on either side of the same connecting rod.
Another possibility which the crank-connecting rod mechanism according to the present invention offers is the adjustability of the compression ratio, which may be important in particular with four-stroke engines. Such adjustability can be achieved by rotating the second crank with respect to the first crank in the top dead center, which makes it possible to change the straight up-and-down movement. of the second crank pin into a more or less oblique movement of the second crank pin. This results in a position change of the bottom and the top dead center. The top dead center will be utilized in the lower part load ranges of the running internal combustion engine, as a result of which the final compression pressure can reach the correct level in spite of the low suction pressure. In the middle part load range up to atmospheric suction, the top dead center will have to be reduced so far that a normal compression ratio is reached.
Adjustment of the crank pins relative to each other can be easily effected when said rotary means comprises of at least two parts, a first part of which is rotatably mounted on the second shaft and fixedly connected to the second crank, and a second part which is in engagement with the first part and which is adjustable with respect to the crankcase. In a practical embodiment thereof, said first part is a gear and said second part is a internal ring gear, with which the first gear meshes. In that case, the ring gear can be adjusted through an angle upon transition to another load range during operation of the crank-connecting rod mechanism so as to adjust the movement of the second crank pin and thus of the bottom and the top dead center.
Another interesting use of the crank-connecting rod mechanism is possible with a four-stroke engine, where it can be used to achieve a different length ratio between the expansion stroke and the compression stroke. This can for example be achieved in that the rotary means are so arranged that the second crank rotates at half the speed, albeit an opposite direction, of the first crank, whereby preferably the first crank is positioned at 0xc2x0 and the second crank is positioned at substantially 90xc2x0 at the uppermost point of the compression stroke, so that the first and the second stroke are both positioned at substantially 180xc2x0 at the end of the expansion stroke. A complete rotary cycle will comprise two revolutions of the first crank in that case, and the ratio between the expansion stroke and the compression stroke can be determined by selecting the first and the second radial distance.
This embodiment has a number of significant advantages:
In the first place it is possible to achieve a long expansion stroke in this manner, so as to maximize utilization of the expansion energy and minimize the pressure upon opening of the exhaust valve. The relatively large expansion ratio will result in an increase of the theoretic efficiency.
Also in this embodiment, the connecting rod makes a much straighter movement during the expansion stroke than usual, as a result of which the guideway forces are significantly reduced. This leads to reduced friction losses and thus to enhanced mechanical efficiency.
Due to the possibilities of using the high-pressure turbine/compressor (up to 3.5 bars) in combination with the adjustable top dead center of the piston, the same engine power can be achieved with a piston displacement of about 30% of that of a conventional engine which does not employ supercharging. As a result, the pumping losses during part load operation of the engine will be much smaller, so that the mechanical efficiency during part load operation is considerably enhanced. In one embodiment, wherein the piston stroke is adjustable, a low compression ratio can be selected when a high boost pressure is used, and a favorable gas exchange can take place, while the compression ratio can be increased and an increased gas residue can be recycled when a low boost pressure is used.
The piston travels at a lower speed at the end of the compression stroke, so that less pre-ignition is required.
Also during part load less pre-ignition is required, because) the final compression pressure remains high, due to the adjustable top dead center position of the piston.
When using a high-pressure turbine/compressor, it is possible to use a relatively small engine, as a result of which the weight of the engine can remain at the same level as that of the current engines in spite of the increased number of engine parts.
Based on the aforesaid advantages, an efficiency enhancement during part load of approx. 50-70% may be possible, while an overall efficiency enhancement of 30-50% can be realised with an average driving style.